1. Technical Field
The present invention relates to a liquid-ejecting head for ejecting droplets from a nozzle in response to the deformation of a piezoelectric element, a liquid-ejecting apparatus, and an actuator that includes a piezoelectric element.
2. Related Art
A representative liquid-ejecting head is an ink jet recording head in which pressure-generating means generates pressure in a pressure-generating chamber to eject ink droplets. For example, the pressure-generating means of the ink jet recording head is a piezoelectric element in which a piezoelectric layer formed of a piezoelectric material having an electromechanical transfer function is disposed between two electrodes. The piezoelectric element is deformed to generate pressure in a pressure-generating chamber to eject ink droplets from a nozzle.
For example, the piezoelectric layer of such a piezoelectric element contains titanium (Ti) and zirconium (Zr) at an element ratio (component ratio) Zr/(Zr+Ti) in the range of 0.5 to 0.8 and is of a monoclinic system (see, for example, JP-A-2007-088446).
The piezoelectric layer may contain titanium (Ti) and zirconium (Zr) and may be of a tetragonal crystal system at a component ratio Zr/(Zr+Ti) of approximately 0.50 (see, for example, JP-A-2005-119166).
In general, it is known that the crystal system of a piezoelectric layer formed of a material having a perovskite structure, such as lead zirconium titanate (PZT), depends on the component ratio of Ti to Zr. For a piezoelectric layer formed of bulk PZT, that is, for a piezoelectric layer under no external stress and free from any restraint resulting from a substrate or the like, it is known that the crystal system of the piezoelectric layer is a tetragonal crystal system at a component ratio Ti/(Zr+Ti) of approximately 0.50 or more (a component ratio Zr/(Zr+Ti) below approximately 0.50) and a rhombohedral or monoclinic system at a component ratio Ti/(Zr+Ti) below approximately 0.50. In other words, in a piezoelectric layer formed of PZT, it is known that the morphotropic phase boundary (MPB) between the tetragonal crystal system and the rhombohedral system is in the vicinity of a component ratio Ti/(Zr+Ti) of 0.50.
The piezoelectric characteristics of a piezoelectric layer vary with the crystal system of the piezoelectric layer, as well as the composition of the piezoelectric layer. As described above, the crystal system of a piezoelectric layer of a piezoelectric element generally depends on the composition of the piezoelectric layer. Although various compositions and crystal systems, such as those disclosed in the patent documents cited above, have been proposed for a piezoelectric layer, the crystal system of the piezoelectric layer always depends on the composition of the piezoelectric layer. The composition of a piezoelectric layer can be restricted by the relationship between the lattice constant of the piezoelectric layer and the lattice constant of an underlying layer. The piezoelectric layer therefore has a limited range of compositions and a limited range of characteristics. This makes it difficult to further improve the deformation characteristics of a piezoelectric element utilizing the piezoelectric characteristics of a piezoelectric layer. Although there has been a demand for further improved deformation characteristics of a piezoelectric element in recent years, it is becoming difficult to satisfy the demand.
Such problems exist not only in piezoelectric elements for use in ink jet recording heads, but also in piezoelectric elements for use in other liquid-ejecting heads for ejecting droplets. In addition, actuators for use in devices other than liquid-ejecting heads also have the same problems.